Near-field ground motion

In recent years, due to the large earthquakes occurred near important and residential areas of the world and the great financial and human losses left, researchers have investigated the effects of different near fault field earthquakes from earthquakes far from the faults.    Near fault field earthquakes have different nature and unique characteristics that may increase the severity of damages and not considering this phenomenon, can increase the damages several times. Therefore, one of the most important and new issues in the seismic design of buildings, bridges, structures with wide spatial distribution and vital arteries, is to pay attention to the vertical component of the earthquakes.Field observations show that many of the damages at short focal lengths, especially in the vertical members of long spans of bridges and buildings, are due to the strong motion of the vertical component.This study has been conducted to consider the effects of the vertical component and the ratio of vertical to horizontal components of near fault earthquakes in Iran due to the importance of the issue and also due to the lack of extensive research in this field according to the Iranian data. In this research, 450 records of near fault field in Iran have been prepared from the Housing and Urban Development Research Center of Iran. Then, 75 records which had pulses with forward directivity, and also their horizontal components had a pulse (perpendicular to the rupture direction) were determined. Using the acceleration response spectrum of these components, the spectral ratio of the vertical component to the horizontal component is determined.The aim of this study is to determine the V/H spectral ratio based on the Iranian database and to provide the relevant relationships for the two categories of rock and soil. Due to the small number of recorded data from earthquakes near the fault and also for considering the local effects of the site, the recorded data on type 1 and 2 (VS ≥ 375 m/s) soils are used as rocky sites and type 3 and 4 (VS ≤ 375 m/s) soils are used as soil sites. Although the acceleration response spectrum of the vertical component of an earthquake is proportional to the acceleration response spectrum of its horizontal component, this ratio is not a fixed number and depends on the various parameters such as period, distance from the fault, soil type, earthquake magnitude and fault mechanism.  According to the results, determining the value of 0.67 for the V/H ratio for two categories in the range of 0-4 seconds is not conservative and it is a small value. Based on the seismic data of Iran, this coefficient in two categories of rock and soil is equal to a value of about 0.8. Therefore, it is recommended to use the V/H spectral ratio proposed in this research in the regulation 2800 (which considers the effects of the vertical component of the earthquake for some structural members), as well as in structures with network distribution and lifelines.